Step required to purchase a product

Step required to purchase a product

Can anyone give me a Step by Step to purchase a product?

Do I install the software before I buy it so it will show up when I add the new Vendor? Because you have to have a product to add a vendor.
Or do I add the software to the product list before I add the new vendor? can't do that because you have to pick a Manufacture from a list that does not have the new manufacture.

I'm really confused about the process 
Please help Thanks
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